In many countries, such as, for example, Italy, France and Germany, there currently exist different activities of development of wide-band satellite communication services based on IP for high-speed trains. In fact, these services are expected to become soon a specific characteristic both of high-speed trains and of traditional ones.
Examples of services that can be exploited by means of a satellite communication system of a train may be:                services based on Voice-over-IP (VoIP) technology;        video-surveillance services;        use of video in streaming, for example on demand;        digital television, for example according to the standard Digital Video Broadcasting-Satellite or Digital Video Broadcasting-Handheld (DVB-S or DVB-H);        Internet netsurfing;        e-mail consultation;        Instant Messaging (IM) services;        consultation of electronic files and/or databases; and        services based on File Transfer Protocol (FTP).        
As is known, on current railway networks of many countries, such as, for example, Italy, various obstacles may be encountered that hinder the visibility of the satellites and that hence can cause discontinuity of the satellite communication services provided on board trains.
In particular, when a train traverses a railway tunnel there is a complete interruption of the direct train-satellite connection.
In order to solve the above problem, it is known to use systems that extend satellite signals into railway tunnels.
A system of the aforesaid type is described in JP2001230718.
In particular, JP2001230718 proposes a satellite communication system for areas in which reception of the satellite signal is poor, the system being based on the use of a satellite receiver apparatus and a transmitter apparatus, which are connected by means of a coaxial cable. Said satellite communication system supports only unidirectional communications, i.e., ones based on signals of a broadcast type, and does not provide solutions to the problem of configuration of mobile terminals that must operate in this context. In addition, JP2001230718 does not propose solutions for switching between the satellite radio channel and the radio channel retransmitted by the satellite communication system.
Similar considerations may be made also as regards the patent application JP2001308765, which proposes a solution for extending a satellite signal of a broadcast type in a tunnel by means of a communications system formed by a satellite receiver antenna positioned outside the tunnel and a plurality of radiotransmitting units inside the tunnel connected by optical fibre.
A further solution for retransmitting signals coming from a satellite to mobile terminals that are located in areas of non-visibility of the satellite is described in WO2007113861.
In particular, WO2007113861 describes a system that comprises a fixed terminal for illumination of a tunnel connected to a fixed satellite station, and a mobile terminal installed on a train and connected to a mobile satellite terminal. In the link from the satellite to the train, the external fixed satellite station receives the satellite signal transmitted by the satellite and directed to the train, and retransmits it into the tunnel through the fixed terminal. In the link from the train to the satellite, the fixed terminal receives the signal transmitted by the train and directed to the satellite, and retransmits it out of the tunnel by means of the external fixed satellite station.
Moreover, once again according to the invention described in WO2007113861, the train is equipped with a transceiver apparatus connected to the mobile satellite terminal which enables exchange of signals directly with the satellite in the periods in which there is visibility between the train and the satellite, i.e., when the train travels in open spaces in which no obstacles are present. When the train exits from an area of visibility of the satellite and enters a tunnel, the system automatically carries out a switching between the satellite channel and the radio channel available in the tunnel, which conveys the same satellite signal but on a different carrier frequency. In a similar way, when the train exits the tunnel and enters an area of visibility of the satellite, the system automatically carries out a switching between the radio channel available in the tunnel and the satellite channel. Switching between channels is carried out in accordance with a logic that processes the information on the quality of the channels and decides which of the two channels to use.
In the system proposed in WO2007113861 the carrier frequencies of the satellite channel and of the radio channel in the tunnel in the satellite-train direction, i.e., in downlink, are mutually linked by a first defined frequency-conversion relationship. Likewise, the carrier frequencies of the satellite channel and of the radio channel in the tunnel in the train-satellite direction, i.e., in uplink, are mutually linked by a second defined frequency-conversion relationship.
The conversions of carrier frequency that the satellite signals undergo both in downlink and in uplink render the system proposed in WO2007113861 somewhat complex and hence rather costly to implement.
Finally, European patent application EP1861530 filed in the name of the Applicant describes a system for extending in railway tunnels navigation satellite signals, such as, for example, GPS (Global Positioning System), GLONASS, or Galileo signals. Said system comprises an antenna positioned outside a tunnel, configured to receive the navigation satellite signals and connected to a plurality of repeaters installed inside the tunnel. The external antenna receives the navigation satellite signals and supplies them to the repeaters which retransmit them inside the tunnel.